1. Field of Invention
The present invention relates generally to the field of command, control, communications, computer, intelligence surveillance, and reconnaissance (C4ISR) hardware and software systems and components, and in particular using spread-spectrum communications.
2. Description of Related Art
TTNT (Tactical Targeting Networking Technology) is an advanced tactical data link currently under development by Rockwell Collins Government Systems and the Advanced Technology Center. Modes supporting Low Probability of Detection (LPD) are a highly desirable addition to existing TTNT functionality. The primary challenge for an LPD receiver is to operate at extremely low signal-to-noise ratio (SNR), often well below negative 20 dB.
Critical to LPD operation is the capability of a receiver to acquire a signal in the presence of chip rate error caused by Doppler shift. This requires acquisition processing that searches over a range of Doppler shifts as well as over a range of arrival times. The present invention presents an apparatus and method for performing a computationally efficient Doppler search for signal acquisition. The reduction in high speed computation enabled by this technique approaches the number of Doppler shifts in the search, which can easily exceed a factor of ten.
The majority of chip rate error observed by an LPD receiver in a tactical environment is a result of Doppler frequency shift. The error Re in chip rate caused by Doppler shift is a function of relative velocity v, nominal (transmitted) chip rate Rc, and the speed of light c: Re=±vRc/c
Because an LPD system must operate at extremely low SNR, the known sequence of chips used for signal acquisition must be very long (on the order of 1 million chips) in order to produce reasonable probabilities of detection and false alarm.
In the present invention, certain terms are used as appreciated by a skilled artisan. Thus “chip” is often defined as “channel bit”. A spread spectrum system, such as used by the present invention, achieves its spectral spreading using one or more techniques such as direct sequence, forward error correction, and orthogonal channel coding. Regardless of the technique used, the bits produced by the spreading are often referred to as “chips”. These chips are modulated and sent over a channel. This distinguishes the bits created by the spreading technique (“chips”) from the information bits going into the spreading technique (“bits”). Note that spread spectrum chips are not required to be binary. “Chip rate” is the rate or frequency at which the chips are transmitted. In a spread spectrum system, the chip rate is much faster than the information bit rate, thus the spectral spreading. “Chip time” is the reciprocal of the chip rate, or the duration in time of a single chip. “Multiple chip times” refers to a period of time that is equal to more than one chip time. A “known sequence” is a sequence of chips (or bits, or symbols) of which an authorized receiver has prior knowledge. The known sequence is typically sent at the beginning of a transmission. The receiver performs a search for the known sequence in order to detect the presence of a desired signal and synchronize its signal processing to it. The process of detecting the presence of a desired signal is often referred to as the signal “acquisition”. After a signal is acquired it can be demodulated to extract information.
In the present invention, due to Doppler shift, the chip rate error caused by Doppler shift can result in multiple chip times of time error over the length of the known sequence. Thus, the LPD receiver must perform signal acquisition that searches over a range of Doppler shifts as well as over a range of arrival times. Furthermore, the signal acquisition function must provide an estimate of the chip rate Doppler shift so that it can be compensated for when demodulating the LPD signal. A conventional solution to this problem is to perform signal acquisition using a correlator for each of a number of possible Doppler shifts. However, the computational resources required for this approach are often prohibitive. What is lacking in the prior art is a computationally efficient technique for solving this problem.